The IEEE (Institute of Electrical and Electronics Engineers) 802.16 standards propose using an Orthogonal Frequency Division Multiple Access (OFDMA) for transmission of data over an air interface. OFDMA has also been proposed for use in 3GPP (Third Generation Partnership Project) Evolution communication systems. In an OFDMA communication system, a frequency bandwidth is split into multiple contiguous frequency sub-carriers, wherein groups of sub-carriers are arranged in logical frequency sub-bands (not necessarily contiguous in frequency), each sub-band comprising multiple orthogonal frequency sub-carriers, that are transmitted simultaneously. A user may then be assigned one or more of the frequency sub-bands for an exchange of user information, thereby permitting multiple users to transmit simultaneously on the different sub-bands. These sub-bands are orthogonal to each other, and thus inter-user and intra-cell interference is minimized.
In order to provide more efficient use of the channel bandwidth, a radio access network (RAN) may transmit and receive data using an antenna array comprising multiple antennas. In the switched antenna system described herein, a customer's premises equipment (CPE), in turn receives and transmits data using one or more of multiple antennas. In an OFDMA system, a serving RAN may beamform a downlink signal for transmission to a CPE via an antenna array and over an associated sub-band. In order to beamform the signal, the RAN maintains a set of weights in association with the CPE and each antenna of the antenna array. When the RAN transmits to the CPE, the RAN applies an appropriate weight, of the set of weights, to the signal applied to each element of the array. In order to determine the set of weights for the CPE, the RAN measures uplink channel conditions in association with the CPE. That is, for any given measuring period, such as a Transmission Time Interval (TTI) (also known as a sub-frame), the CPE transmits a pre-determined symbol to the RAN in a sub-band allocated to the CPE by the RAN. Based on a comparison of the symbol received to the symbol that the RAN knows was transmitted, the RAN is able to estimate channel conditions for the CPE in the allocated sub-band and determine a set of weights for beamforming a downlink transmission to the CPE in the sub-band.
When a multi-antenna CPE implements a switched antenna system for transmit diversity, the channel conditions may vary depending upon which antenna, of the multiple antennas of the CPE, is used by the CPE to transmit to the RAN. As a result, the beamforming implemented by the RAN may become sub-optimal when the CPE switches antennas subsequent to the channel sounding that was most recently used, by the RAN, to determine the beamforming weights.
Accordingly, there is a need for a method and apparatus for controlling switching of a switched antenna system at a CPE in an OFDM system such that the switching does not disrupt beamforming at a serving RAN.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.